In induction heating, an inductor is excited to oscillate in the medium frequency range. This inductor is conventionally integrated by means of an additional capacitance in a so-called oscillating load circuit that is excited by an inverter, for example, by adding voltage pulses close to the resonance frequency of the oscillating circuit using a bridge circuit, semi-bridge circuit, or using one single switch.
To this end, a mains voltage, for example, a single phase or multi-phase AC voltage from a voltage supply, is usually rectified and smoothed, and the DC voltage is supplied to an inverter that excites the oscillating load circuit. This configuration generates a large portion of harmonics in the current at the mains voltage supply connection. The harmonics are generated substantially through rectification and smoothing of the rectified voltage rather than by the inverter. Or, alternatively, the portions generated by the inverter can be easily filtered, since the inverter usually operates at considerably higher frequencies than the mains voltage, that is, in a range from a few kHz to some MHz.
To avoid this disadvantage, either passive filter circuits or active power factor correction (PFC) members are conventionally interconnected. Both circuits are expensive and also very heavy since they require large inductances. Moreover, these circuits require a large amount of space.
Induction ovens for airplanes are disclosed, for example, in DE 198 18 831 A1. The excitation configurations for such ovens must be light and have very narrow restrictions concerning harmonics.
The mains voltage in airplanes is 200 volts measured from phase to phase in a three-phase system that is operated at a frequency of, for example, 400 Hz. The PFC members can be operated in this region only with inductances that must be specially produced and are therefore relatively expensive and heavy. A passive filter element requires even more complex inductances (because of size, weight, and cost considerations).
It is possible to use voltages with unregulated frequencies (for example, in a range up to about 800 Hz) in airplanes. If implemented, such a design will render the use of PFC members even more complicated. Moreover, the weight and costs of the excitation configurations will also increase.
FIG. 5 shows a conventional excitation configuration. The voltage of each phase P1, P2, P3 is rectified relative to the neutral conductor N with a single bridge rectifier 11, 21, 31, respectively. Each of the DC voltages generated in this manner is supplied to a flyback converter 13, 23, 33, and each flyback converter is controlled by a PFC controller 12, 22, 32, respectively. Each PFC controller 12, 22, 32 ensures that a largely sinusoidal current is taken from the mains connection, thereby minimizing the harmonic wave portions that act on the mains. Each of the AC output voltages from the flyback converter 13, 23, 33 is rectified again using a rectifier 14, 24, 34, respectively, and is then supplied to a common DC-link voltage circuit U4. The DC-link voltage circuit U4 can be adjusted by driving the flyback converters 13, 23, 33, thereby controlling the power and energy supply of the oscillating load circuits. A common inverter 41 is connected to the DC-link voltage circuit U4. The conventional excitation configuration also includes one or more capacitances 43 and inductors 15, 25, 35 for induction heating of the food, both of which are integrated in the oscillating load circuits.
In some conventional excitation configurations, two inductors 15, 25 are used for direct heating of the food trays and a third inductor 35 is connected for heating water to generate water vapor. Such a configuration is described in DE 198 18 831 A1. The oscillating load circuit for generating water vapor is often not required, and if it is included, is usually not used for as long as the other load circuits. In this configuration, therefore, it should be possible to disconnect the inductor 35 from the oscillating load circuit. To this end, a relatively complex switch 42, which should be bipolarly operated, is required. However, this switch 42 is expensive and heavy, thus adding to the overall cost and weight of the unit that houses the entire configuration. For example, the unit around the three flyback converters 13, 23, 33 is very heavy since it requires coils with large ferrites, and is very complex and expensive.